This is a new R21 from Dr. Mansfield that aims to determine the basis for virulence in African trypanosomes. The proposed research would extend studies originally funded in a one year RO3 award, made in 1997 but extended to two years via a no-cost extension. The aims of the previous proposal are essentially unchanged in this one, however, additional progress is reported. Dr. Mansfield hypothesizes that virulence may be controlled by a rheostat-like mechanism involving differentially expressed virulence genes (or genes that inhibit virulence). There are two specific aims: first to identify differentially transcribed mRNAs and synthesized proteins in genetically related members of the LouTat 1 clone of Trypanosoma brucei rhodesiense and to characterize the genomic organization of these genes. Second, the role of these genes in virulence would be confirmed by genetic manipulation of trypanosomes using gene complementation, disruption and over expression.